1. Field of the Invention
The present invention relates to an image reader utilizing a CCD line sensor or the like and, more particularly, to the improvement in quality of image reading by the image reader.
2. Description of the Prior Art
In FIG. 1 there is shown general arrangement of an image reader utilizing a CCD line sensor. An original 2 is placed on a table 4. A light source (not shown) is provided on the underside or topside of the table 4. Light from the light source which has passed through the original 2 or which has been reflected from the original 2 is projected through an optical system 6 onto a line sensor 8 to form an image thereon. The line sensor 8 has photoelectric conversion elements (CCD) arranged in line configuration therein, each photoelectric conversion element being capable of outputting a signal tuned to the density of the original 2 at the corresponding position therein.
The table 4 is driven by a motor 10 to move in a secondary scanning direction S. This movement is carried out for individual primary scan lines thereby to sequentially derive density signals until such signals for the entire original 2 are obtained.
The configuration of the CCD line sensor 8 is shown in FIG. 2. The CCD line sensor 8 has twelve photoelectric conversion elements P.sub.1 -P.sub.12, arranged in line configuration. Electric charge is accumulated in photoelectric conversion elements P.sub.1 -P.sub.12 correspondingly to the quantity of light sensed thereby. The charges accumulated in odd n-th elements P.sub.1, P.sub.3, P.sub.5, P.sub.7, P.sub.9, P.sub.11 are derived from an output 16 via a first output route comprising a transfer path T.sub.0 and an amplifier A.sub.0. The charges accumulated in even n-th elements P.sub.2, P.sub.4, P.sub.6, P.sub.8, P.sub.10, P.sub.12 are derived from the output 16 via a second output route comprising a transfer path T.sub.E and an amplifier A.sub.E.
Output delivery is initiated upon application of a line start clock signal from a terminal 14. Subsequently, a transfer clock signal is input from a terminal 12, whereupon the charges in the transfer paths T.sub.0, T.sub.E are transferred to the respective amplifiers. Thus, charges in the transfer paths T.sub.0, T.sub.E are sequentially drawn in alternate order. That is, reading is effected in order of photoelectric conversion elements P.sub.12 to P.sub.1.
Provision of two transfer paths as stated above is intended to improve the rate of reading. Transfer of charge through a transfer path takes time. In view of this fact, two transfer paths are provided so that the transfer of charge through the one transfer path may be completed while the charge in the other transfer path is in the course of being transferred, whereby the rate of reading can be increased.
However, prior art image readers of the above-described type involve the following problems.
As stated above, CCD line sensor 8 is provided with two transfer routes (each comprising a transfer path and an amplifier). It is difficult to arrange that the two transfer routes are completely identical in electrical characteristics. Therefore, output Q.sub.0 of odd n-th elements and output Q.sub.E of even n-th elements are different from each other, as shown in FIG. 3A, when spots of same density are read. In order to correct such difference, it is common practice to arrange so that outputs of odd n-th and even n-th elements are electrically equalized in both reference black and reference white levels. This level equalization is generally known as shading correction. Even with such shading correction, however, some divergence in middle density level is involved between outputs Q.sub.0 and Q.sub.E, as shown in FIG. 3B.
The fact that output Q of odd n-th elements and output Q.sub.E of even n-th elements are of different levels as stated above has been a cause of density unevenness as often seen in dot duplicated images output on the basis of sensed images. This problem of density unevenness is explained hereinbelow.
FIG. 4 shows the condition of read outputs in the case where an original of even density is read by a CCD line sensor having such characteristics as shown in FIG. 3B. Since the original has no variation in density, outputs of same level should be drawn from the photoelectric conversion elements of the CCD line sensor if the conversion elements are of same characteristics. However, because of the fact that odd n-th elements of the CCD line sensor are characteristically different from even n-th elements as shown in FIG. 3B, there occurs some difference in density between reading by the former elements and reading by the latter elements. Such condition is illustrated in FIG. 4, in which shaded portions are greater in value of density reading than unshaded portions. That is, outputs of even n-th elements P.sub.2, P.sub.4, P.sub.6, P.sub.8, P.sub.10, P.sub.12 are greater in density value than outputs of odd n-th elements P.sub.1, P.sub.3 P.sub.5, P.sub.7, P.sub.9, P.sub.11.
When dots are formed on th basis of outputs of a CCD line sensor of the type shown in FIG. 4, the following problem does occur. Assume, for example, that the size of one dot constituting one component of a dot unit corresponds to 5.5.times.5.5 pieces of photoelectric conversion elements. In this case, one dot D.sub.1 of a dot matrix DOT depends on the sum of outputs d.sub.1 of 5.5.times.5.5 pieces of photoelectric conversion elements as to whether it is on or off (i.e., black-dotted or not). Likewise, one dot D.sub.2 of the dot matrix DOT depends on the sum of outputs d.sub.2 of 5.5.times.5.5 pieces of photoelectric elements as to whether it is on or off.
A comparison between outputs d.sub.1 and outputs d.sub.2 shows that outputs d.sub.2 include outputs of even n-th elements in a larger proportion than outputs d.sub.1 ; therefore, the former presents higher density. Thus, columns m.sub.1 -m.sub.7 of the dot matrix DOT are classified into two groups, one of which includes columns m.sub.3, m.sub.6 which are likely to go "on," the other including columns m.sub.1, m.sub.2, m.sub.4, m.sub.5, m.sub.7 which are likely to go "off." As a consequence, there occurs a problem that duplicated images have a vertical stripe pattern defect (output irregularity) formed thereon.